1. Field of the Invention
This invention relates to solar energy conversion systems, and more particularly to a solar energy conversion system having improved efficiency achieved by concentrating and refracting electromagnetic radiation incident on a flat array of adjacent lenses.
2. Description of the Prior Art
It is well know in the art to convert solar energy to electricity using semiconductor photovoltaic cells. To improve efficiency of conversion, optical concentrators may be employed to increase the intensity or brightness of the sunlight that strikes the photovoltaic cell. Furthermore, the broad solar spectrum may be separated into photons having different energies and directed to appropriate photovoltaic cells having suitable band gaps.
A solar energy converter is disclosed in U.S. Pat. No. 2,949,498 to Jackson (1960) that splits the solar spectrum by stacking photovoltaic cells. A high band gap photovoltaic cell is placed in front of one or more photovoltaic cells having successively lower band gaps. High energy photons are absorbed by the first cell and lower energy photons are absorbed by the following cell. This method is disadvantageous in that the leading cells interfere with the following cells.
Borden et. al., Proceedings of the Fifteenth IEEE Photovoltaic Specialists Conference, pp. 311–316 (1981), describes a design in which light is incident upon a dichroic filter that transmits high energy photons to a high band gap photovoltaic cell and reflects low energy photons to a low band gap cell. This method is disadvantageous in that a single dichroic filter yields only two spectral components, and an additional dichroic filter is needed for each additional desired spectral component.
Ludman et al., Proceedings of the Twenty-fourth IEEE Photovoltaic Specialists Conference, pp. 1208–1211 (1994), describes a design in which the spectrum is split by diffraction, and different photovoltaic cells are arranged to capture light of different wavelengths. A hologram serves as the diffraction grating and also concentrates the sunlight. This method is disadvantageous in that it is difficult to economically create durable diffraction gratings having high optical efficiency over a wide portion of the solar spectrum.
While refractive dispersion is a well known means of separating light into its spectral components, it is not trivial to create a refractive optical arrangement that is suitable for solar energy conversion. For example, refractive dispersion designs using only a single array of prisms or a concentrator with a single dispersing prism at or near its focus do\not simultaneously provide adequate dispersion and concentration. U.S. Pat. No. 4,021,267 to Dettling discloses a spectrum splitting arrangement comprising concentrating, collimating, and refractive dispersing means. This method is disadvantageous in that the collimating optical element introduces additional transmission losses and alignment difficulties.
In U.S. Pat. No. 6,015,950 to Converse (1997), a “Refractive Spectrum Splitting Photovoltaic Concentrator System”, the system uses two optical elements and does no have auto-focusing bands on the solar cells. Sunlight passes sequentially through two separated arrays of refracting elements which direct photons in different energy bands to appropriate solar energy converters, such as semiconductor photovoltaic cells having suitable band gaps.
A solar concentrator utilizing a primary optical concentrator is disclosed in U.S. Pat. No. 6,399,874 to Olah (2002). A Fresnel lens has a predetermined focal distance and the photovoltaic cell is supported within a housing within the focal range of the lens and at a distance from the lens that is less than the focal distance.
A rainbow type of photovoltaic array equipped with light-concentrator and spectral beam splitter optics is disclosed in “Advanced Rainbow Solar Photovoltaic Arrays” available from NASA's Jet Propulsion Laboratory at www.nasatech.com/briefs/june03/NP021051.html. The rainbow photovoltaic array comprises side-by-side strings of series-connected photovoltaic cells. The cells in each string have the same band gap, which differs from the band gaps of the other strings. To obtain maximum energy conversion efficiency and to minimize the size and weight of the array for a given sunlight input aperture, the sunlight incident on the aperture is concentrated, then spectrally dispersed onto the photovoltaic-array plane, whereon each string of cells is positioned to intercept the light in its wavelength band of most efficient operation. A proposed unitary concentrator/spectral-beam-splitter optic includes a parabolic curved Fresnel-like prism array with panels of photovoltaic cells on two sides. A surface supporting the solar cells can be adjusted in length or angle to accommodate the incident spectral pattern. The proposed concentrator/spectral-beam-splitter optic suffers the disadvantage of being hard to focus, and costly to build versus a single flat lens.
Notwithstanding the known problems and attempts to solve these problems, the art has not adequately responded to date with the introduction of a solar energy conversion system which improves efficiency by concentrating and splitting the solar energy spectrum with a system that is simple, efficient and affordable.